Constraints on frequency assignments

Satellite systems share the frequency spectrum with other satellite systems and, in most frequency bands, with terrestrial radio systems also. Consequently interference arises. In order to keep the inter­ference down to acceptable levels, constraints on the characteristics of systems which determine their liability to cause and suffer inter­ference have been agreed. Also, assignments are registered interna­tionally; a new user of a frequency defers to a registered established user under the jurisdiction of another country where there is no international agreement to the contrary. It is assumed that inter­ference problems between systems falling within the jurisdiction of the same country will be resolved by the government of that country. The constraints and agreements vary from service to service and from band to band; the more important provisions are as follows.

In the frequency bands at 12GHz in which frequency assignment plans have been agreed for satellite broadcasting (ITU, 1990g) and in the bands at 14.5GHz andl 8 GHz where there are corresponding feeder link assignment plans (ITU, 1990h), limits have been agreed for all of the satellite and earth station characteristics and all of the emission and orbital parameters which significantly affect the level of interference that these systems can cause to one another. With very minor exceptions, the radio stations of other services which also have allocations in these bands are not permitted to cause significant interference to satellite broadcasting, and they must accept any interference which they receive from authorised satellite broadcasting.

In the frequency bands at 4.5, 7.0, 10.8, 11.3 and 13GHz, which were used in drawing up the frequency and orbital slot allotment plan which provides for one fixed satellite system per country (ITU, 1990b), constraints were placed on equipment characteristics and emission and orbital parameters in more flexible ways than were used for satellite broadcasting at 12GHz. There are also procedures to be used when systems are set up, using these allotments, to ensure that these systems do not interfere with one another. However, these frequency bands are also allocated for terrestrial radio services and the measures outlined below to limit interference between terrestrial and satellite systems in bands which have not been planned for the satellite service apply in the allotment plan bands also.

These two sets of frequency plans for satellite systems are limited to geostationary satellites. Most communication satellites operating in other bands are also geostationary, but a few use other orbits. Satellites in non-geostationary equatorial orbits would present a persistent interference hazard for geostationary satellites. Non-geostationary satellites in orbits inclined to the equator, which pass through the equatorial plane twice in every orbit, could cause and suffer intermittent interference. Since it is agreed that the geosta­tionary satellite orbit is the most important, especially for the fixed satellite service, there is a regulation (RR 2613) that requires sys­tems using non-geostationary satellites using fixed satellite allocations must cease operation whenever this is necessary to prevent interference to a geostationary satellite system (ITU, 1990i).

There are also agreed procedures, implemented under the super­vision of the ITU, to ensure that the owners of proposed new geostationary satellite systems, and the national administrations under whose jurisdiction they fall, will meet with corresponding representatives of established geostationary systems belonging to other countries, to make sure that interference problems will not arise; this is called frequency co-ordination (ITU, 1990j). When frequency co-ordination has been successfully completed and the new system has been brought into operation, frequencies assigned to the satellite can be registered in the Master International Fre­quency Register, maintained by the ITU. Procedures intended to deal systematically with the risk of interference between non-geos­tationary satellites have not yet been developed.

To keep within acceptable limits the interference which satellite transmitters cause at the receiving stations of terrestrial radio links, upper limits have been placed on the spectral power flux density (PFD) which satellites, regardless of orbit, may set up at the Earth's surface in frequency bands shared with terrestrial services (ITU, 1990k). These limits vary with frequency and with the angle of elevation at which the satellite signal reaches the ground. However, a limit of-145dB relative to 1 watt per m² in a sampling bandwidth of 4kHz is typical for frequency bands below 15GHz. Correspond­ing limits apply to the power of terrestrial transmitters operating in bands in which satellite receivers operate.

Finally there is the problem of interference between earth stations operating with satellites and the transmitting and receiving stations of terrestrial radio systems. As usual, national governments must resolve problems arising between stations under their jurisdiction. However, if a new earth station could suffer unacceptable inter­ference from a foreign terrestrial transmitter or vice versa, the extent of the problem must be determined and solutions found where necessary before frequency assignments made to the earth station may be registered internationally (ITU, 19901).

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